JP2003337462A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus by which stable image density and image quality are attained by accurately detecting the surface resistance of a film medium remarkably fluctuated by moisture absorption and the resistance fluctuation of a member related to transfer such as a transfer roller at a manufacturing time or at a long time in use and always applying transfer bias matched to a resistance value. <P>SOLUTION: In the case that a sheet is made of a material consisting of the film of a resin or the like as a base material, the transfer bias applied to a first transfer part is set to have a specified fixed value, and the transfer bias applied to the second and succeding transfer parts is decided based on the detected value of a transfer current including an electrostatic charge which is detected by the first detecting means of the first transfer part and with which the sheet is previously electrified by a pre-transfer electrifying means. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus utilizing an electrophotographic method, etc., and particularly, in a color image forming apparatus having a plurality of transfer portions, the uniformity and stability of the image formation on a film medium such as OHT. It is about improvement.

[0002]

2. Description of the Related Art Conventionally, various systems such as an electrophotographic system, a thermal transfer system and an inkjet system have been adopted as an image forming apparatus. Among these, the image forming apparatus using the electrophotographic method is superior in high speed, high image quality, and quietness.

FIG. 6 shows a schematic structure of an example of an image forming section of a conventional image forming apparatus using an electrophotographic system. The electrophotographic image forming apparatus has, for example, a drum-shaped electrophotographic photosensitive member that is rotatable in the arrow direction, that is, a photosensitive drum 101 as an image bearing member, and the surface thereof is the primary charging unit 1
After 02 is uniformly charged, an exposure means 111 such as an LED or a laser exposes it according to image information to form an electrostatic latent image on the surface of the photosensitive drum 101. afterwards,
The developing device 108 electrostatically attaches a developer (including toner) to this electrostatic latent image, and a toner image is formed on the photosensitive drum 101. Next, from the feeding cassette 115, the feeding roller 1
The sheet P is conveyed to a transfer position facing the photoconductor drum 101 by the 16 and the conveying means 117, 118 and the registration roller pair 119, and the toner on the photoconductor drum 101 is transferred onto the sheet P by the action of the transfer roller 104 which is the transfer means. The image is electrostatically transferred.

Thereafter, the unfixed toner image transferred onto the sheet is heated and pressed by the fixing device 121 to be fixed onto the sheet, and a permanent image is formed. On the other hand, the transfer residual toner remaining on the photosensitive drum 101 after the transfer is removed by the cleaning blade, and the waste toner container
Recovered within 110. The photosensitive drum 1 whose surface is thus cleaned is repeatedly used for image formation.

Further, in recent years, color image forming apparatuses using an electrophotographic system have become widespread. The color image forming apparatus using the electrophotographic system is also divided into various systems. For example, in addition to the well-known multiple transfer system and intermediate transfer body system, a multiple development system in which a color image is superposed on the surface of the photoconductor and then collectively transferred to form an image, or an image of a plurality of different colors is formed. There is an in-line method or the like that has a forming unit (process station) and transfers the developer to a sheet conveyed by a conveyor belt.

The color image forming apparatus of the in-line system has many advantages such as high speed and advantageous image quality because the number of toner image transfers is small. In this in-line method, a configuration is proposed in which process stations are arranged in a vertical direction and sheets are conveyed almost vertically in order to improve usability and reduce an installation area.

FIG. 7 is a block diagram showing the essential parts of a conventional full-color image forming apparatus using an in-line system. In the figure, in the image forming apparatus, an electrostatic transfer conveyance belt (hereinafter referred to as a conveyance belt 114) as a transfer unit is provided with a driving roller 12
3, the driven roller 122 and the tension rollers 113a and 113b are wound around each roller. Photosensitive drums 101a to 101d carrying toner images of cyan, yellow, magenta, and black are arranged along the peripheral surface of the conveyor belt 114, and the conveyor belt 114 is composed of transfer rollers 104a to 104d as transfer means. The toner images are sequentially and superposedly transferred onto the sheet P that is adsorbed and carried on the sheet P to form a color toner image on the sheet P.

A transfer power source 125a is attached to the transfer rollers 104a to 104d.
The transfer bias is applied according to the steps from to d. Further, in the pre-transfer charging unit, the pre-transfer charging roller 120 as a pre-transfer charging unit faces the driven roller 122 via the transport belt 114, and is configured to sandwich the transport belt 114 and the sheet P. A voltage is applied to the pre-transfer charging roller 120 by a pre-transfer charging power supply 127, which is a high voltage power supply, so that a charge is applied to the sheet P, and the charged sheet P is charged.
The carrier belt 114 by polarizing the carrier belt 114
Is electrostatically adsorbed by.

In the in-line type image forming apparatus shown in FIG. 7, a sheet having a very high resistance value such as a resin film (OHT: OverHead Transparency) for an overhead projector is used as the sheet P, and an image is output to this sheet. In that case, if you gradually increase the transfer bias so that it is not affected by the previous process station,
In the final station, there was a problem that the bias value was extremely high so that the photosensitive drum 1 was damaged by dielectric breakdown. Further, even if the bias value does not damage the photosensitive drum 1, the higher the bias value is, the more expensive the power supply device that can output the bias value becomes, and the cost of the device increases. .

Film media such as OHT is a high resistance film such as PET coated with a charge control agent on the surface layer.
Particularly, the surface resistance value greatly changes depending on the dry state of the charge control agent. For example, in a low temperature and low humidity environment (hereinafter L /
In the L environment, which is typified by 15 ° C x 10% RH), the surface resistance (resistivity) of OHT rises to the 12th power (LogΩcm), and the high temperature and high humidity environment where OHT adsorbs moisture (hereinafter H / H environment) , 30 ℃ x 80
% RH), the surface resistance of OHT is 9th power (LogΩcm)
To a certain extent. In this way, under the environment where the surface resistance of the OHT decreases, the charge due to the transfer bias received at the previous station escapes from the end of the OHT, and the state in which the potential at the center of the OHT and the potential at the end differ is the last station. However, there is a problem in that it is accumulated in the image and appears as a large image unevenness.

In order to solve this problem, it has been hitherto disclosed in Japanese Patent Laid-Open No.
6-27837, JP-A-11-161035, etc., the sheet is pre-charged to the opposite polarity to the charge-up generated by the transfer bias by using the pre-transfer charging means, and the transfer bias of the first station is very low, or It has been proposed to start with a polarity opposite to the normal transfer bias and keep the transfer bias at the final station low.

For example, in the in-line type image forming apparatus shown in FIG. 7, when an image is formed by passing OHT paper under H / H environment, if the bias applied to the pre-transfer charging roller 20 is 0.5 kV,
Transfer bias is 0.75kV, 1.5k from the first station
V, 2.25kV, 3.0kV, and image unevenness occurs due to high bias in the final station. Therefore, the pre-transfer charging roller 20 is used as the pre-transfer charging means, the applied bias is set to 1.5 kV to pre-charge the sheet, and the transfer bias is sequentially set from the first station to -0.5 kV, 0.25 kV, 1.0 kV.
The toner image is transferred to the sheet with V and 1.75 kV. As described above, by reducing the absolute value of the transfer bias of the final station, it is possible to reduce the unevenness of the potential accumulated on the OHT and prevent the unevenness of the image.

Further, conventionally, a configuration has been proposed in which the pre-transfer charging means detects the resistance of the OHT. As shown in FIG. 7, the pre-transfer charging power supply 127 for applying the pre-transfer charging bias to the pre-transfer charging roller 120 is connected to the second detecting means 128, and the output of the pre-transfer charging power supply 127 is controlled by constant voltage. If the charging bias is applied, the pre-transfer charging power source 12
When the current value flowing through 7 is controlled by a constant current, the voltage value generated in the pre-transfer charging power source 127 when the charging bias is applied can be detected.

Similarly, a transfer power source 125a for applying a transfer bias to the transfer roller 104a of the first process station.
Is connected to the first detection means 126, and the transfer power source 125a
If the output of C is controlled by a constant voltage, the current value flowing through the transfer power supply 125a when the transfer bias is applied is set.
It is possible to detect the voltage value generated in each.

Further, detection means 126, 128, pre-transfer charging power source 12
7 and the transfer power supplies 125a to 125d of each station are the CPU 12
It is connected to 9 (arithmetic control mechanism), and detecting means 126, 12
The outputs of the transfer power supplies 125a to 125d and the pre-transfer charging power supply 127 can be arbitrarily controlled according to the detection result at 8.

A method of controlling the transfer bias will be described below by taking as an example the case where the pre-transfer charging power supply and the transfer power supply of each station are controlled to a constant voltage.

As described above, the sheet P is fed from the feeding cassette 115 or the like to the feeding belt 114 via the feeding roller 116, the feeding means 117 and 118, and the registration roller pair 119. At this stage, a preset detection bias is applied from the pre-transfer charging power supply 127 to the pre-transfer charging roller 20, the current value flowing at that time is detected by the second detecting means 128, and the detected current value is stored in the CPU 129. Send to and process. This second detection means 1
It is possible to set various process conditions such as the pre-transfer charging bias and the transfer bias based on the result of detecting the current value at 28.

To set the detection bias, a bias value may be set in advance for each sheet type, and an appropriate bias value may be selected and applied according to the sheet type, or the second detection means 128 may be used. Based on the detection result before sheet feeding,
A bias value suitable for the sheet may be calculated by the CPU 129, and may be feedback-controlled and applied.

In this way, based on the application of the detection bias at the pre-transfer charging roller 20 and the resulting current value,
The CPU 129 determines the type and characteristics of the sheet, and calculates and sets the set values of various process conditions such as the pre-transfer charging bias value (preliminary charging bias value) and the transfer bias value which are optimal according to the type and characteristics of the sheet. Is possible. If necessary, it is possible to obtain the electric resistance value of the sheet.

[0020]

However, as shown in the above-mentioned conventional example, when the resistance of the OHT is detected by the pre-transfer charging means, the resistance fluctuation due to the variation in the production of the transfer roller and the so-called energization deterioration due to the durability are caused. Since it is difficult to accurately separate the resistance variation caused by the influence of the surface resistance and the interference amount between the pre-transfer charging bias and the transfer bias caused by the surface resistance variation of OHT, it may be necessary to determine the optimum transfer bias value in some cases. There was a problem that I could not do it.

Further, the surface layer of OHT absorbs moisture in a high humidity environment,
Since a current is passed through the surface layer, the transfer current interferes not only between the pre-transfer charging unit and the transfer unit but also between the transfer units. Therefore, for example, when the transfer is being performed from the photoconductor drum 101c, the photoconductor drum 101b in front of
When the trailing edge of the HT comes off, the transfer current in the photoconductor drum 101c becomes temporarily unstable, and horizontal streaky unevenness may occur in the image. This phenomenon has a problem that the resistance of the transfer roller is increased due to the deterioration of energization, and it becomes noticeable immediately after the continuous durability of the apparatus.

Therefore, as conventionally proposed in Japanese Patent Application No. 2001-397197, for example, the transfer including the transfer roller is performed at the time of powering on the main body and at the time of a cleaning sequence executed when returning from the sleep state. The resistance of the member is detected at the transfer unit, the resistance detection result of this member is compared with the resistance value of the sheet detected by the pre-transfer charging unit, and the correction amount is calculated before the transfer is performed. There is a means for applying the corrected transfer bias when reaching the transfer portion.

However, although this means is very effective for ordinary paper, for film media such as OHT in which the resistance of only the surface layer fluctuates greatly due to moisture absorption,
Since the resistance detection value of the sheet fluctuates greatly, it is difficult to correlate with the resistance fluctuation of the member, and it is very difficult to correct the transfer bias correctly and predict the interference amount between transfer parts to control the transfer bias. It was difficult.

Therefore, according to the present invention, the surface resistance of the film medium, which greatly changes due to moisture absorption, and the resistance fluctuation that occurs during manufacturing or durability of a member related to transfer such as a transfer roller are accurately detected, and the resistance value is constantly measured. An object of the present invention is to provide an image forming apparatus that achieves stable image density and image quality by applying a suitable transfer bias.

[0025]

In order to solve the above-mentioned problems, a typical structure of an image forming apparatus according to the present invention has a plurality of transfer portions for transferring a toner image to a sheet, and a sheet carrier for supporting the sheet. A body, a pre-transfer charging unit that charges the surface of the sheet to a polarity opposite to the normal charging polarity of the toner according to the type of the sheet, and a first transfer unit that first reaches the sheet to detect current or voltage. In the image forming apparatus having one detection means, when the sheet is made of a material such as a resin film as a base material, the transfer bias applied to the first transfer portion is set to a predetermined fixed value, and the first transfer is performed. The transfer bias to be applied to the transfer parts subsequent to the second transfer part is determined based on the detected value of the transfer current including the charging charge precharged on the sheet by the pre-transfer charging part detected by the first detection part of the second transfer part. thing And it features.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of an image forming apparatus according to the present invention will be described with reference to the drawings. 1 is a schematic wiring diagram of a pre-transfer charging bias and a transfer bias circuit of the image forming apparatus according to the present embodiment, FIG. 2 is a schematic wiring diagram of a first transfer portion, FIG. 3 is an explanatory diagram of a transfer current detection circuit, and FIG. Control time chart of pre-transfer charging bias and transfer bias, and FIG. 5 is a schematic configuration diagram of a full-color image forming apparatus using an in-line system according to the present embodiment.

(Overall Configuration of Image Forming Apparatus) First, the overall configuration of the image forming apparatus will be described with reference to FIG. In the present embodiment, in the image forming apparatus, an electrostatic transfer conveyor belt (hereinafter referred to as a conveyor belt 14) as a transfer unit includes a driving roller 23, a driven roller 22, tension rollers 13a and 13b.
It is wound around each roller.

Along the peripheral surface of the conveyor belt 14, process stations 24a to 24d, which are image forming units for cyan, yellow, magenta, and black, are arranged, and the conveyor belt 14 extends in the direction of the arrow in the figure. By rotating, the sheet P is sequentially conveyed to each of the process stations 24a to 24d. Each of the process stations 24a to 24d is a process cartridge that can be attached to and detached from the main body of the image forming apparatus.
a to d, developing devices 8a to d, cleaning devices 10a to 10d
It is configured to be integrated together. Further, the apparatus main body is provided with exposure means 11a to 11d corresponding to the process stations 24a to 24d for forming electrostatic latent images of respective colors.

The photosensitive drum 1 in each process station has a transfer roller 4 as a transfer means via a conveyor belt 14.
It is in contact with a to d. The sheet P is sent out from the feeding cassette 15 or the like into the image forming apparatus by the pickup roller 16 and the feeding rollers 17 and 18. Then, the registration roller pair 19 conveys the image in synchronism with the image forming operation and the conveyance of the sheet, and guides the sheet P to the conveyance belt 14 to the pre-transfer charging section where the pre-transfer charging is performed.

In the pre-transfer charging section, a pre-transfer charging roller 20 as a pre-transfer charging means faces the driven roller 22 via the conveyor belt 14 and sandwiches the conveyor belt 14 and the sheet P. A charge is applied to the sheet P by applying a voltage to the pre-transfer charging roller 20, and the charged sheet P is electrostatically adsorbed to the transport belt 14 by polarizing the transport belt 14.

The sheet P thus attracted to the transport belt 14 sequentially passes through each process station, and the toner images of the respective colors on the photosensitive drums 1a to 1d are successively transferred. Then, the fixing device 21 heats and pressurizes the color image of the unfixed toner to form a permanent image.

The transport belt 14 has a thickness of 50 to 200 μm,
Resin film such as PVDF (vinylidene fluoride resin), ETFE (tetrafluoroethylene-ethylene copolymer resin), polyimide, PET (polyethylene terephthalate), polycarbonate, etc. with a volume resistivity of the 9th power to 16th power (LogΩcm), or A rubber base layer having a thickness of about 0.5 to 2 mm, such as EPDM, is coated with a urethane resin in which a fluororesin such as PTFE is dispersed, for example.

The transfer belt 14 does not normally carry a toner image directly on the surface thereof, so that it is less likely to be contaminated by the toner image, but at the time of jamming, the background fog toner adheres to the non-image portion, or When a system is used in which a registration mark or a density detection pattern is directly formed on the conveyor belt 14 to detect it, toner adheres to the conveyor belt 14. A separate cleaning unit is provided to clean the contaminated toner, or the transfer roller 4a is used.
The process of applying a cleaning bias having a polarity opposite to that at the time of transfer to the cleaning devices 10a to 10d via the photosensitive drums 1a to 1d to the cleaning devices 10a to 10d. It is possible to do about d.

The sheet P, which has undergone the transfer process, is fixed by a fixing device.
The toner image is conveyed to 21 and heated and pressurized there, and the toner image is fixed on the surface of the sheet as a permanent image.

(Pre-Transfer Charging Bias and Transfer Bias Circuit) Next, the pre-transfer charging and transfer bias circuit will be described with reference to FIG.

A pre-transfer charging positive bias circuit 35 is connected to the pre-transfer charging roller 20 and supplies a high voltage to the pre-transfer charging roller 20 provided in contact with the conveyor belt 14 to transfer the sheet P to the conveyor belt. The sheet is electrostatically adsorbed on 14 and the sheet is charged as a pre-charging means for assisting transfer. Here, the pre-transfer charging roller 20 is included in the attraction load 34, and the attraction load 34 also includes the transport belt 14, the sheet P, the driven roller 22, and the like. Further, the pre-transfer charging positive bias circuit 35 is capable of variably outputting a high voltage according to the ON width of the PWM clock signal S32 from the control circuit 28, and the control circuit 28 controls the environmental conditions in which the image forming apparatus is used. The pre-transfer charging voltage of the pre-transfer charging positive bias circuit 35 is controlled according to the paper type, the paper type, the print timing, and the like.

When a predetermined high voltage is output from the pre-transfer charging positive bias circuit 35, most of the accompanying charging current I32 is generated by the pre-transfer charging roller 20, which is the attraction load 34, and the conveyor belt.
14, the sheet P and the driven roller 22 flow to the ground. The charging current I32 flowing to the ground is the second detection means 3
6. Follow the current loop that returns to the pre-transfer charging positive bias circuit 35 via the pre-transfer charging negative bias circuit 37.

The second detecting means 36 is a circuit for converting the charging current I32 into an analog voltage and transmitting the charging current signal S35 to the control circuit. The details of the operation are the same as those of the first detecting means 31, and will be described later.

A pre-transfer charging negative bias circuit 37 is also connected to the pre-transfer charging roller 20, and is turned on / off at a predetermined timing by an on / off signal S33 from the control circuit 28 to adhere to the pre-transfer charging roller 20 surface. It is used to clean the generated waste toner.

The transfer positive bias circuits 30a to 30d are connected to the transfer rollers 4a to 4d for the four colors, respectively, and the toner images of the respective colors formed on the photosensitive drum 1 are transferred to the conveyor belt.
In order to sequentially superimpose and transfer onto the sheet conveyed on 14,
A bias is applied to the shaft of each transfer roller 4. Here, the transfer roller 4 is included in the transfer load 29, and the transfer load 29 also includes the transport belt 14, the sheet P, and the photosensitive drum 1a.
The transfer positive bias circuits 30a to 30d are similar to the pre-transfer charging positive bias circuit 35 in that the PWM clock signal S28 from the control circuit 28 is used.
It is possible to variably output a high voltage according to the ON width of a to d.
Control the output voltage of a to d.

As shown in FIG. 2, when a predetermined high voltage is output from the transfer positive bias circuit 30a of the first transfer portion (cyan color), most of the transfer current I28a for the first color due to the output of the predetermined high voltage is transferred to the transfer load. 29a transfer roller 4a, conveyor belt
14, the sheet P, the photoconductor drum 1a, and the ground. The transfer current I28a for the first color that has flowed to the ground is
A current loop returning to the transfer positive bias circuit 30a via the second detection means 36 and the transfer negative bias circuit 32a is followed. Similarly, the transfer currents I28b-d flow in the second to fourth transfer portions.

Further, transfer negative bias circuits 32a and 32c are connected to the transfer rollers 4a and 4c, so that they can be variably controlled by the PWM signal S29 of the control circuit 28 like the transfer positive bias circuits 30a to 30d. And is mainly used for cleaning the waste toner adhering to the surface of the conveyor belt 14.

In this embodiment, the first transfer portion (cyan color)
And only the transfer bias circuit of the third transfer portion (magenta color) has a transfer negative bias. During the cleaning sequence of the conveyor belt 14, for example, the first,
Approximately -1500V to the transfer rollers 4a and 4c in the third transfer section
The waste toner charged to the negative polarity by applying
Approximately +1500 V is applied to the transfer rollers 4 of the fourth transfer portion, and the positively charged waste toner is attached to the surface of the photosensitive drum 1 paired with each transfer roller 4. The waste toner attached to the surface of each photoconductor drum 1 is
It is scraped off by a rubber-like cleaning blade 9 provided so as to be in contact with the upper surface of the photosensitive drum 1, and is housed in the cleaning device 10.

Further, as shown in FIG. 2, the first transfer portion is provided with the first detection means 31, and the transfer current I28a flowing through the first transfer portion when the transfer bias is output is converted into an analog voltage. The charging current signal S30 is transmitted to the control circuit.

The operation of the first detecting means 31 using the operational amplifier 41 will be described with reference to FIG. Transfer current I28a
Flows from the ground of the operational amplifier 41 of the current detection circuit to the output terminal, the protection resistor 43, and the resistor 39 as shown by the arrow in the figure. The + terminal of the operational amplifier 41 is connected to the ground by the impedance matching resistor 42. Since the + terminal and-terminal of the operational amplifier 41 are generally imaginary short-circuited, the-terminal can be considered to be the ground potential. When the transfer current I28a flows, a Vi voltage is generated with reference to the virtual ground of the negative terminal of the operational amplifier 41. This Vi is expressed by the following equation when the transfer current is Itr and the resistor 39 is Ra.

Vi≈Itr × Ra Therefore, the first detection means 31
The value of the output signal S30 from the above becomes Vi, and as a result, the first detecting means 31 converts the magnitude of the transfer current I28a flowing through the first transfer portion into an analog voltage.

Output signal S3 output from the first detection means 31
0 is transmitted to the A / D port in the control circuit 28. Control circuit
28 digitally converts the transfer signal value based on the output signal S30.

When an image is formed on a film medium such as an OHT, the image forming speed is reduced to a low speed such as 1/2 to 1/4 together with the fixing device in order to make the toner fixability sufficient. Sometimes. In the case of an image forming apparatus in which the speed of fixing and the speed of image formation are the same, this speed change needs to be performed prior to image formation. As a sheet detection unit to be executed before image formation, a transmissive sensor including a light emitting unit and a light receiving unit is provided in the feeding unit, and when the sheet is fed, a transmissivity higher than a certain level when the sheet passes through the sensor is provided. If the sheet is detected, the sheet is determined to be OHT, and the speed of subsequent image formation can be reduced. When a resin-based sheet such as a non-transparent gloss film is used, it is possible to set conditions such as image forming speed based on the sheet information set by the user.

(Control of Pre-Transfer Charging Bias and Transfer Bias) In the image forming apparatus having the above-mentioned configuration, Table 1
Table 2 shows the results of examining the value detected by each detector and the voltage value when the optimum image was obtained while changing the environment, the endurance state of the device, and the OHT moisture absorption state.

[0050]

[Table 1]

Each item in Table 2 is roughly classified into a detection item and a transfer bias setting, and details thereof are as follows.

5 μA adsorption constant current OHT resistance detection result (V):
This is the voltage value obtained when the OHT paper is passed by controlling the voltage value so that the pre-transfer charging bias is a constant current of 5 μA. Here, the value of 5 μA is a value selected in the image forming apparatus used in the present embodiment in order to charge the OHT by a necessary and sufficient amount before transfer even when conditions such as the environment change.

OHT surface resistance measurement value (square): Under each environmental condition, the surface resistance (Log Ωcm) was measured for OHT in a state of absorbing moisture under the same condition.

Pre-rotation transfer resistance detection (μA): A voltage of 1 kV is applied to the transfer section to drive the transfer roller while the apparatus is operating and the sheet is not present in the transfer section when the main body is powered on. It is a current value as a result of detecting the resistance of a member involved in transfer including.

Pre-rotation adsorption resistance detection (V): The pre-transfer charging bias is set to a constant current of 13 μA in a state where the sheet is not present in the pre-transfer charging means while the apparatus is operating, such as when the main body is powered on. It is a voltage value as a result of controlling the voltage value.

Transfer cyan current (μA): At the time of image formation with OHT, the pre-transfer charging bias is subjected to constant voltage control at the voltage value obtained as a result of constant current control, and then transferred to the cyan transfer portion, which is the first transfer portion. It is the current value detected by applying a constant voltage of -250V. Since the efficiency of cyan transfer depends on the amount of pre-transfer charging, the transfer bias can be maintained at a constant value regardless of the conditions. Also, as mentioned above,
By keeping the bias of the black transfer portion, which is the fourth transfer portion, low, it is possible to prevent image unevenness that occurs in black. Therefore, it is desirable to keep the cyan bias value as low as possible and, if possible, keep it in the negative direction. But on the other hand,
If the cyan bias is set too low, there arises a problem that the density of cyan is lowered and unevenness caused by leakage of charges due to pre-transfer charging from the edge of the sheet becomes significantly visible in cyan. Therefore, in this embodiment, the transfer bias of the cyan transfer portion, which is the first transfer portion, is set to -250V.

Transfer bias setting (for each color): The pre-transfer charging bias is subjected to constant voltage control at a voltage value obtained as a result of constant current control, and the cyan transfer portion is controlled at -250V constant voltage.
Under the conditions of A to H, it is the optimum bias value with which the optimum image density and image uniformity are obtained for each color of yellow, magenta, and black.

[0058]

[Table 2]

The detection items in Table 2 are compared with the transfer bias setting, and the detection items that can guide the transfer bias setting will be examined. Comparing the detection items in Table 2 with the transfer bias settings, the transfer bias settings gradually increase toward the bottom of the table, while the detection items have a correlation with the transfer bias settings. It may or may not be obtained.

5 μA adsorption constant current OHT resistance detection result (V):
In this study, there is a certain correlation between the detected value and the transfer bias setting. However, when the resistance or durability of the pre-transfer charging roller is changed, the detection result changes regardless of the transfer setting (see CE). Therefore, it is not desirable to use it as an index for setting the transfer bias. Or
It is necessary to use another detection result together so as to reflect the resistance of the pre-transfer charging roller.

OHT surface resistance measurement value (power): There is a good correlation between this measurement value and the transfer bias setting. However, it goes without saying that this measured value cannot be used for setting the transfer bias because the surface resistance of the OHT cannot be measured in the apparatus.

Pre-rotational transfer resistance detection (μA): This detection value well reflects the resistance state that changes with the durability and environment of the transfer roller, but does not reflect the OHT state at all (see B and D). , Cannot be used alone to set the transfer bias.

Pre-rotation adsorption resistance detection (V): This value has almost nothing to do with the resistance or state of the transfer system (see B to E).
It cannot be used to set the transfer bias. However, there is a possibility that it can be used as correction data for other detection results as a means for detecting the state of the pre-transfer charging roller.

Transfer cyan current (μA): As described above, the transfer bias of the cyan transfer portion (first transfer portion) is fixed to -250V in this embodiment. On the other hand, for example, even in the same 27 ° C / 70% environment, the transfer member under the condition C was not used at all, and the resistance increased due to the long-term use under the condition E, and the resistance temporarily increased even after continuous energization. The optimum transfer bias condition is largely different from the state in which the resistance is increased. Therefore, when the current flowing in the cyan transfer portion was measured in this state, it was a detection value that reflected the OHT state and the state of the transfer member and had a good correlation with the transfer bias.

This transfer cyan current is not a current obtained by simply making the transfer bias negative, but the current of the interference component which flows from the pre-transfer charging bias to the cyan transfer portion through the surface of the OHT is dominant. Conceivable. Since this interference current is a value determined not only by the surface resistance of the OHT but also by the resistance of the transfer roller, it can be a very good index for setting the transfer voltage after the second transfer portion.

FIG. 4 shows a control time chart of the pre-transfer charging bias and the transfer bias according to this embodiment. OHT
5μA after rushing into the nip part of the charging roller before transfer (T1)
Is used as a target current to control the pre-transfer charging bias with a constant current. When the predetermined time has passed and the constant current control becomes stable (T
In 2), the average voltage value is calculated, and the pre-transfer charging bias is switched to constant voltage control based on this value (T3). At this time, as the constant voltage value of the pre-transfer charging bias, the detection value itself may be used, or a voltage value different from the detection value may be applied using a conversion table or the like.

Next, after the OHT plunges into the cyan transfer nip portion (T4), the transfer bias of the cyan transfer portion (first transfer portion) is controlled to a constant voltage value of -250V. At a time point (T5) when the constant voltage control becomes stable after a lapse of a predetermined time, the average value of the current values flowing in the cyan transfer portion is calculated, and the transfer bias value after the second transfer portion is determined based on this value. The transfer bias value is, for example, the average value of the current flowing in the cyan transfer portion shown in Table 2 (transfer Cyan current in the table) and the transfer bias value after the second transfer portion corresponding to that value (the optimum bias value in the table). Ye
llow, optimal bias value Magenta, optimal bias value Black)
Is stored in a nonvolatile storage medium as a table,
The transfer bias value after the second transfer portion is determined by selecting the value in the table that is closest to the average value of the current values actually flowing in the cyan transfer portion. The data stored in the table is not limited to that shown in Table 2, and can be transferred more finely (or coarsely).
The Cyan current value and the corresponding optimum bias value after the second transfer portion may be stored. In addition, the relationship between the average value of the current flowing in the cyan transfer portion and the optimum bias value after the second transfer portion is approximated by a function, and the average value of the current flowing in the cyan transfer portion is substituted into the function to obtain the optimum bias. The value may be calculated. The nonvolatile recording medium described above is
It is desirable to be provided in an engine controller (not shown) that controls each part of the image forming apparatus such as the control circuit 28.

By controlling the pre-transfer charging bias and the transfer bias as described above, there are variations in the resistance of the environment in which the device is placed, the OHT moisture absorption state, the manufacturing lot, etc., and during the manufacturing and durability of the members involved in transfer. It is possible to always control the optimum OHT pre-charging bias and transfer bias in response to all of the factors such as resistance variation due to deterioration, and it is possible to always form high-quality images.

Further, in the conventional method of detecting the resistance value of the sheet at the transfer portion, the transfer bias value at the time of detecting the resistance value and the transfer bias value after the detection are different. However, the bias control is not stable and the detection tends to be unstable. However, in the present embodiment, a constant minus 25 is always applied to the first transfer unit that performs the detection.
Since 0 V is applied, there is no restriction on the detection position, and it is possible to perform detection with high margin and high accuracy.

Second Embodiment Next, a second embodiment of the image forming apparatus according to the present invention will be described. The same parts as those of the first embodiment will be designated by the same reference numerals and the description thereof will be omitted.

In the first embodiment, the transfer bias control for the OHT uses only the transfer current value detected in the cyan transfer portion (first transfer portion), but in the present embodiment,
In addition to the control of the first embodiment, the detection value of the constant current control of the pre-transfer charging bias is used particularly in a low humidity environment.

In Table 2, the 5 μA adsorption constant current OHT resistance detection result is compared with the transfer cyan current.
In Fig. 2, it is understood that the transfer cyan current hardly reflects the influence of the environmental change. This is in a low humidity environment
It is considered that this is because the surface resistance of the OHT increases and the current interference from the pre-transfer charging bias does not occur with respect to the cyan transfer bias. On the other hand, it can be seen that the resistance increase due to OHT drying can be detected by the constant current control of the pre-transfer charging bias.

In the first embodiment, in particular, the decrease in the surface resistance of the sheet and the increase / decrease in the interference current due to the fluctuation in the resistance of the transfer roller are very important for the transfer bias setting for preventing the interference between the transfer parts after the cyan transfer part. Therefore, it was useful to control the transfer bias based on the result of measuring the interference current at the cyan transfer portion. However, in a low humidity environment, not only the pre-transfer charging unit and the cyan transfer unit, but also the transfer units do not interfere with each other,
It is not necessary to determine the transfer bias based on the interference current. Therefore, measuring the resistance value of the sheet by the pre-transfer charging means is most useful as an index for determining the transfer bias.

From the above, in the present embodiment, 5 in Table 2 is used.
μA adsorption constant current OHT resistance detection result F value (3000
V) as a threshold value, and when a voltage value higher than this is detected, the voltage values of all the transfer parts after cyan are determined at that time, and the transfer bias value is set based on the detected value of the transfer cyan current. Only when it is less than the value of F without being determined, the voltage value of the transfer portion after the second transfer portion is determined based on the detected value of the transfer cyan current. The transfer bias value is, for example, 5 μA adsorption constant current OHT resistance detection result shown in Table 2 is 350.
The optimum bias value from the first transfer portion to the fourth transfer portion when it is 0 V (the optimum bias value Cyan of G in the table, the optimum bias value Yellow, the optimum bias value Magenta, the optimum bias value B
lack) is stored as a table in a non-volatile storage medium, and when the actually detected 5 μA adsorption constant current OHT resistance detection result is larger than 3000 V, the values of the stored table are used. 4. Determine the transfer bias value for the transfer section. The data stored in the table is not limited to the value in the case of 3500V in Table 2, and more finely stores the 5 μA adsorption constant current OHT resistance detection result and the corresponding optimum bias value from the first transfer unit to the fourth transfer unit. You may stay. Also, 5 μA
The relation between the adsorption constant current OHT resistance detection result and the optimum bias value from the first transfer part to the fourth transfer part is approximated by a function, and the optimum bias value is obtained by substituting the adsorption constant current OHT resistance detection result of 5 μA into the function. May be calculated. The above-mentioned nonvolatile recording medium is preferably provided in an engine controller (not shown) that controls each part of the image forming apparatus such as the control circuit 28.

As described above, it is possible to always select the optimum transfer bias by dividing the detection unit of the transfer bias control depending on the presence or absence of the interference current due to the decrease in the surface resistance of the OHT. Even when using such a special sheet, it is possible to always obtain stable high-quality images.

[0076]

Third Embodiment A third embodiment of the image forming apparatus according to the present invention will be described. The same parts as those of the first embodiment will be designated by the same reference numerals and the description thereof will be omitted.

The current flowing into the photosensitive drum 1a when the sheet P is in contact with both the pre-transfer charging roller 20 and the transfer roller 4a of the first transfer portion and when the sheet leaves the pre-transfer charging roller 20. The amount is different. As a result, a phenomenon occurs in which the image density decreases in the latter half of the sheet from the moment the sheet P leaves the pre-transfer charging roller 20. In order to make this phenomenon inconspicuous, the transfer bias may be controlled so that the same current as before flows even after the sheet P leaves the pre-transfer charging roller 20.

As a method for determining the timing at which the sheet P leaves the pre-transfer charging roller 20, there is a method for calculating sheet size information. In the present embodiment, the transfer bias is shifted by calculating the timing at which the trailing edge of the sheet leaves the pre-transfer charging roller 20 based on the sheet size information set by the user using the printer controller.

In this embodiment, Tables 1 and 2 of the first embodiment are used.
Under the same conditions as described above, as shown in Table 3, the transfer bias value at the trailing edge of the sheet in the first transfer portion was set based on the detected value of the transfer cyan current.

[0080]

[Table 3]

As shown in Table 3, by increasing the transfer bias of the first color at the timing when the trailing edge of the sheet leaves the pre-transfer charging roller 20, the density unevenness before and after the sheet P leaves the pre-transfer charging roller 20 is increased. I was able to get rid of.

Further, by varying the correction amount based on the detected value of the transfer cyan current, the optimum correction amount can always be obtained in accordance with the moisture absorption of the OHT and the resistance value of the transfer roller 4, and uniform image density can be obtained. Can be obtained.

In this embodiment, since the transfer current is measured at the leading end of the sheet at the first transfer portion, the interference current of the actual pre-transfer charging bias with respect to the first transfer portion can be measured. is there. Therefore, it is possible to very accurately correct the increase or decrease in the transfer current depending on the presence or absence of the interference component at the rear end of the sheet that does not overlap the front end of the sheet on which the transfer current detection is performed.

In each of the above-mentioned embodiments, the OHT is used as an example of the film medium. However, for example, G sold by Canon Sales Co., Ltd.
The present invention can be applied to a gloss film such as F-2.

At this time, for example, when the type of sheet can be selected on the computer screen on the side of transmitting image information, such as a printer, normal paper and OHT or gloss film modes are separated. As a result, it is possible to select the first detecting means and the table for setting the transfer bias, which are suitable for the respective characteristics. Further, in an apparatus such as a copying machine in which the main body of the image forming apparatus has a sheet selection function, the mechanism of the main body is used to switch the mode, and the sheet resistance detection method and the bias are detected based on the information provided by the control unit. It is possible to select the setting. It is also possible to equip the apparatus body with a sheet type detection mechanism so that the first detection means and transfer bias setting can be selected.

In each of the above embodiments, a negative bias value (-250V) is set for the cyan transfer portion, which is the first transfer portion located at the most downstream side in the transport direction, and the second transfer portion is used. A positive bias value is given to the fourth transfer section, but a negative bias value given to the cyan transfer section is set to a lower value (for example, -500V), and a bias value given to the second transfer section is also negative. The bias value may be a positive bias value (for example, -250V), and the positive bias value may be applied to the third transfer portion and the fourth transfer portion thereafter. Even in this case, the bias value is set to increase from the first transfer portion toward the fourth transfer portion, but the bias value given to the first transfer portion is low, so the bias value given to the fourth transfer portion should be kept low. Therefore, it is possible to always obtain a uniform and high-quality image without unevenness without using an expensive power source.

[0087]

As described above, in the image forming apparatus according to the present invention, the fluctuation of the resistance value caused by the manufacturing of the sheet or the environment where the apparatus is placed, the resistance fluctuation during the manufacturing of the transfer roller, It is possible to always perform optimum transfer bias control irrespective of resistance fluctuation due to endurance, and it is possible to always obtain a uniform and high-quality image without unevenness. Also, by making it possible to select the most suitable detection means depending on the type of sheet and environmental conditions, it is extremely adaptable,
The transfer bias control can be performed with high accuracy, the adaptability of the apparatus to the sheet can be improved, and the cost and function of the apparatus can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic wiring diagram of a pre-transfer charging bias and a transfer bias circuit of an image forming apparatus.

FIG. 2 is a schematic wiring diagram of a first transfer portion.

FIG. 3 is an explanatory diagram of a transfer current detection circuit.

FIG. 4 is a control time chart of pre-transfer charging bias and transfer bias.

FIG. 5 is a schematic configuration diagram of a full-color image forming apparatus using an inline system.

FIG. 6 is a diagram illustrating a schematic configuration of an example of an image forming unit of a conventional image forming apparatus.

FIG. 7 is a diagram showing a configuration diagram of main parts of a conventional full-color image forming apparatus.

[Explanation of symbols]

P ... Sheet 1 ... Photosensitive drum 2 ... Primary charger 4 ... Transfer roller 8 ... Developer 9 ... Cleaning blade 10… Cleaning device 11 ... Exposure means 13a ... Tension roller 13b ... Tension roller 14… Conveyor belt 15… Feed cassette 16… Pickup roller 17… Feeding roller 18… Feeding roller 19… Registration roller pair 20… Pre-transfer charging roller 21… Fixing device 22 ... driven roller 23… Drive roller 24… Process Station 28… Control circuit 29… Transfer load 30 ... Transfer positive bias circuit 31… First detection means 32… Transfer negative bias circuit 34… Adsorption load 35 ... Pre-transfer charging positive bias circuit 36… Second detection means 37… Pre-transfer charging negative bias circuit 39 ... resistance 41… operational amplifier 42 ... Resistance for impedance matching 43… Protection resistance 101 ... Photosensitive drum 102 ... Primary charging means 104… Transfer roller 108… Developer 110… Waste toner container 111… Exposure means 113… Tension roller 114… Conveyor belt 115… Feed cassette 116… Feeding roller 117… Transportation means 118… Transportation means 119 ... Registration roller pair 120… Charging roller before transfer 121… Fixing device 122 ... driven roller 123… Drive roller 125… Transfer power supply 126… First detection means 127… Pre-transfer charging power source 128… Second detection means 129 ... CPU

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2H200 FA18 GA02 GA06 GA07 GA12                       GA23 GB41 JA02 JA28 JA29                       JA30 JB07 JB10 NA02 PA04                       PA06 PA10 PB02 PB05 PB12                 2H300 EA01 EB04 EB07 EB12 ED05                       ED07 EF06 EF08 EH16 EJ01                       EJ09 GG01 GG11 PP02 PP16                       QQ05 QQ10 QQ26 RR02 RR07                       RR12 TT04 TT06

Claims (7)

[Claims] 1. A plurality of transfer portions for transferring a toner image onto a sheet, a sheet carrier for supporting the sheet, and a pre-transfer for charging the surface of the sheet to a polarity opposite to the regular charging polarity of the toner according to the type of the sheet. In an image forming apparatus having a charging unit and a first detection unit for detecting a current or a voltage provided in a first transfer portion where the sheet first arrives, the sheet is made of a resin-based film as a base material. In this case, the transfer bias applied to the first transfer unit is set to a predetermined fixed value, and the sheet includes pre-charged by the pre-transfer charging unit detected by the first detection unit of the first transfer unit. An image forming apparatus characterized in that a transfer bias applied to a transfer portion after the second transfer portion is determined based on a detected value of a transfer current. 2. The image forming apparatus according to claim 1, wherein the type of the sheet is detected by measuring the transparency of the sheet to be fed after the sheet feeding operation is started. 3. The image forming apparatus according to claim 1, wherein the sheet type is detected based on information provided by a control unit of the image forming apparatus. 4. The image according to any one of claims 1 to 3, wherein a bias having a reverse polarity to a bias applied to a transfer portion after the second transfer portion is applied to the first transfer portion. Forming equipment. 5. The bias according to any one of claims 1 to 3, wherein a bias having a polarity opposite to that of a bias applied to the transfer portions after the third transfer portion is applied to the first to second transfer portions.
The image forming apparatus according to the item. 6. When the detection is performed by the first detection unit of the first transfer unit, a bias having a reverse polarity to the bias applied to the transfer units subsequent to the third transfer unit is applied to the first transfer unit, When an image is formed on a sheet having a size larger than the distance from the pre-transfer charging unit to the first transfer unit in the sheet conveying direction, the first end of the sheet passes the pre-transfer charging unit, and 4. The image formation according to claim 1, wherein the bias applied to the first transfer portion is switched based on the detection value of the transfer current detected by the first detection portion of the one transfer portion. apparatus. 7. A second detection unit for detecting a voltage applied to the pre-transfer charging unit, wherein the bias of the pre-transfer charging unit is controlled with a constant current,
The voltage obtained when the sheet is passed by the second detecting means is detected, and the bias of the pre-transfer charging means is switched to a constant voltage value determined based on the detected voltage value, and a voltage value higher than a certain threshold value. 2. When the is detected, the transfer bias value of the transfer portion is also determined based on the detection value of the pre-transfer charging means, and the transfer bias value is not determined based on the current detection result of the transfer portion. The image forming apparatus according to any one of 6 above.